With the development of portable devices such as personal computers and cell phones in recent years, demand is increasing for batteries for use as a power source for the devices. The batteries for such an application are required to operate at room temperature as well as to have a high energy density and excellent cycle characteristics.
To satisfy the above demand, batteries including, as a negative electrode active material, a material containing silicon (Si) or tin (Sn), which can provide a very high capacity, are regarded as promising. Examples of the material containing Si or Sn include a simple substance of Si or Sn, an oxide of Si or Sn, and an alloy containing Si or Sn.
However, the materials listed above undergo a change in crystal structure upon absorption of lithium, so their volumes increase significantly. In other words, the materials listed above undergo a significant change in volume during charge and discharge. For this reason, repetition of charge and discharge causes a contact failure between the active material and the current collector, resulting in a short charge/discharge cycle life.
In order to solve this problem, for example, WO 01/029912 proposes to form a silicon thin film on a current collector having a roughened surface.
However, according to the technique disclosed in WO 01/029912, there is not a space inside the Si thin film, and the current collector is made of a copper foil of a single composition. Thus, a large stress, which is generated during the expansion of the active material, is transmitted to the current collector, causing the Si thin film to detach from the interface between the Si thin film and the current collector, or causing the electrode plate to deform.